A conventional general purpose lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. These two aberrations can be reduced, for example, by using multiple lenses in which each lens has a unique index of refraction and dispersion. Spherical aberration can also be corrected for a single lens with the correct choice of aspheric surface.
It is well known in optics that aberrations in optical systems can be reduced by employing lens elements with a spatially varying index of refraction. These GRadient INdex elements are termed GRIN elements. The material for the fabrication of such gradient lenses can be made by a variety of processes such as SOL-GEL, infusion, and diffusion and may be glass, plastic or other suitable optical material. The two most common index geometries discussed are the `axial` gradient, in which the index is constant in a two dimensional plane but varies in the direction perpendicular to the plane, and the `radial` gradient, in which the index is constant in circular cylinders around the optical axis, but varies as a function of the radius.
The physics of the propagation of light rays in a medium with a gradient index of refraction is well understood. For example, the trajectories of the rays can be calculated analytically in many cases, see, for example, the paper by Michael E. Harrigan, "Some first-order properties of radial gradient lenses compared to homogeneous lenses", pp. 2702-2705, Applied Optics, Vol. 23, No. 16, August 1984. Designs for useful optical systems that utilize radial gradient optical elements have been produced. Among these see J. Benschop and J. Braat, "Gradient-index objectives for CD applications", pp. 1195-1200, Applied Optics, Vol. 26, No. 7, April 1987, and H. Nishi, H. Ichikawa, M. Toyama and I. Kitano, "Gradient-index objective lens for compact disk system", pp. 3340-3344, Applied Optics, Vol. 25, No. 19, Oct. 1986.
The possible use of an anamorphic GRIN lens in which the surfaces of constant index (isoindicial surfaces) are elliptical cylinders around the optical axis has been discussed by J. M. Stagaman and D. T. Moore, "Laser diode to fiber coupling using anamorphic gradient-index lenses", pp. 1730-1734, Applied Optics, Vol. 23, No. 11, June 1984. To the present inventors' knowledge, GRIN material with such a geometry has not been commercially available.
Most commercially available optical design software programs provide the designer with appropriate software tools to design with GRIN elements. Exact calculations of the gradient properties needed to achieve a required performance of such a lens can be performed by several commercially available optical design software packages. Examples of commercially available software that offer GRIN design capabilities include "Code V" available from Optical Research Associates of Pasadena, Calif., "Synopsis" from BRO, Inc. of Tucson, Ariz., and "ZEMAX" from Focus Software, Inc. of Tucson, Ariz. These lens design codes give the designer many choices for gradient types, glass types, and chromatic models.
For a general introduction to the structure of the eye and its optical properties, see Chapter 15, entitled "Vision", in Applied Optics, A Guide to Optical System Design, Vol. 2, by Leo Levi, John Wiley & Sons, New York, (1968) and Chapter 1, entitled "The Eye and Vision" by Glenn A. Fry, in Applied Optics and Optical Engineering, Vol. 11, edited by Rudolf Kingslake, Academic Press, New York, (1965).
A standard contact lens has a homogeneous index of refraction and has spherical surfaces. It will be hereafter abbreviated as an HCL. The optical power of the lens is chosen to correct the error in the focal length of the natural eye of the wearer. Since the index of refraction is constant throughout the lens, the parameters available to the optical designer are the index and anterior radius of curvature, RA, of the lens. The posterior radius of curvature, RP, of the lens must be chosen to closely match that of the anterior (front) surface of the eye. The optical power of a homogeneous lens arises only from the curved exterior surfaces of the lens, the index of refraction, and to a small extent, the thickness which typically is made as thin as possible to maximize O.sub.2 transport to the cornea, and CO.sub.2 from the cornea. This allows the eye to breath and is a factor in comfort, extended wear, and health.
A conventional contact lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. The chromatic aberrations are normally not very noticeable or distracting to the user because the eye can change focus rapidly and the brain can process the information. Spherical aberration however is quite noticeable and bothersome to the wearer; the increased blur size, due to spherical aberration, cannot be compensated for by focus or information processing. The two lens system consisting of the conventional homogeneous contact lens together with the lens in the eye cannot eliminate the spherical aberrations in ophthalmic applications. This is due to the lack of parameters that are at the disposal of the optical designer. A homogeneous CL with spherical surfaces can correct for refractive errors in the eye, a first order error in optical power, but there are no degrees of freedom left to correct for aberrations. Therefore, there is need for an improved system in which these aberrations can be reduced.
Under low light level conditions, the iris diaphragm of the eye increases in diameter. This increase in aperture lets more light in but leads to increased spherical aberration even in the standard, or emmetropic (20/20), eye. The same effect occurs in an eye corrected for refractive errors in focal length, or power, using a homogeneous CL. Spherical aberration results in an increased spot diameter at the retinal focus and therefore results in a loss of visual fidelity. This loss in fidelity results in a loss of contrast sensitivity and a severe degradation in visual acuity. Therefore there is need for an improved system in which this loss in visual fidelity can be ameliorated for both the normal and corrected eye.
One of the properties of a contact lens is that it tends to move around in the eye, or drift, as it is worn. The optical axis of the eye and the axis of the CL are not always coincident. This leads to a loss of visual fidelity manifesting from an apparent defocus. Therefore there is need for an improved system in which deleterious effects of the drift in position can be reduced.
A contact lens can be worn for only a finite time. This time may vary from individual to individual and is due to a lack of gaseous, (O.sub.2 and CO.sub.2), exchange between the corneal surface of the eye and the atmosphere. This exchange depends upon the material used for the CL and also upon the thickness of the lens. In general, the thinner the lens, the better the gaseous transport between cornea and air. For lenses of positive power, the edge thickness of the HCL is less than the center thickness. For negative power lenses, (so-called minus lenses), the edge thickness of the HCL is greater than the center thickness. The gaseous transport of a minus lens decreases as a function of aperture because of the increased edge thickness. For positive power lenses, (plus lens), the overall HCL diameter is limited because of the diminished edge thickness and/or the center thickness of the lens must be increased. Therefore there is need for an improved contact lens which can be as thin as possible and with a relatively constant thickness over the aperture, but still perform the required optical functionality.
The present manufacturing techniques used to fabricate contact lenses are reasonably cost effective. However, there is always a need for an improved contact lens design which can be manufactured by a simpler and cheaper process. An HCL is commonly made by two different methods. A circular flat disk, called a button, is produced of the selected material. In the first method, this button is put in a lathe and the front and rear surface are carved to the specified radius of curvature and then polished. In the second method, the button is placed on a spherical surface with a specified radius of curvature, heated until it deforms to the shape of the surface, thereby forming one of the surfaces of the completed CL, and then this `warped` button is lathed to cut the other optical surface.
It is an object of the invention to provide a contact lens system utilizing a radial and/or a combination of radial and axial gradient index of refraction, herein termed a conical gradient. One essential component of the invention is to develop a description of the optical parameters of an optically imperfect, or ametropic, eye and then use it in the design of the RGRIN contact lens parameters so as to optimize the performance of the combined optical system.
It is still another object of the invention to provide a contact lens system which improves the contrast sensitivity and resolution of the eye under low light level conditions. Such an improvement in the performance of the eye is useful even for individuals that have emmetropic, i.e., (20/20), vision under bright light conditions. This is accomplished by reducing spherical aberration.
It is yet another object of the invention to provide a contact lens system which can correct the eyesight of the wearer if astigmatism is present. In this case the correction is made by using a CL with an elliptical index of refraction profile.
It is still another object of the invention to provide a contact lens system which can manufactured by a simplified fabrication process.
It is a further object of the invention to provide a bifocal correction in a single RGRIN CL by use of an index profile that is not monotonically rising or falling.
It is still another object of the invention to provide a contact lens system which improves the contrast sensitivity and resolution of the eye of the wearer with near-sightedness, (Myopia)
It is still another object of the invention to provide a contact lens system which improves the contrast sensitivity and resolution of the eye of the wearer with far-sightedness, (Hyperopia)